Nonuniformly treating articles and supporting them therefor

ABSTRACT

In making transistors using lead frames, improved bonds and savings in gold result from plating gold more heavily and in controlled amounts at the ends of the lead frames where wire and semiconductive chip bonds are to be made. To accomplish such nonuniform plating, the lead frames are arranged in a spaced, radial, fan-like manner about an axis to form a cylindrical configuration of the frames. The ends of the frames where bonds are to be made are on the outer periphery of the configuration. The carrier strips, which are located opposite such ends and which are eventually trimmed away, are on the internal portion of the configuration and may be partially masked to reduce gold plating on such strips. A cathode is connected to the lead frames, which are then immersed in a plating bath and revolved about the axis of the frames and within a hollow cylindrical anode spaced midway between two flat anodes. A sparger pumps the bath in a direction substantially perpendicular to the axis of the frames. The frames and cylindrical anode are positioned midway between the top surface of the bath and the bottom of a tank holding the bath. With this arrangement the frames are nonuniformly plated, with more gold being plated on the ends where bonds are to be made than on the carrier strips. Also, each group of three leads of each frame for a single transistor is plated substantially alike. This arrangement of the frames and rotation of them in the bath also results in the plating of a large number of frames using a minimum volume of a tank holding the bath. An increase in plating rates is also achieved.

United States Patent 1 91 Primary ExaminerRobert L. Bleutge Attorney,Agent, or FirmR. Y. Peters Rackus et al. 5] June 4, 1974 NONUNIFORMLYTREATING ARTICLES ['57] ABSTRACT AND SUPPORTING THEM THEREF R In makingtransistors using lead frames. improved [75] Inventors. John JosephRackus9 Whitehall; bonds and savmgs 1n gold result from platmg gold JohnPaul skflbeck, Allentown; more heav1ly and 1n controlled amounts at theends of Alfons Henryk szkudlapski, the lead frames where w1re andsemrconductrve chip Bethlehem, an of Pa. bonds are to be made. Toaccomplish such nonuniform plating, the lead frames are arranged in aspaced,

[ Asslgneei western Elecmc p y radial, fan-like manner about an axis toform a cylinlncmptratedo New York drical configuration of the frames.The ends of the [22] Filed. July 17, 1972 frames where bonds are to bemade are on the outer periphery of the configuration. The carrierstrips,

l l PP which are located opposite such ends and which are Re'atedApplication Data eventually trimmed away, are on the internal portion[60] Continuation-impart of Ser. No. 263 897 June 19 of theconfigurallon and may t?e paruany masked to I972 abandoned which is adivision of Ser. No: reduce gold platmg on Such A cathode ls coll-100i") Dec 21 1970 PM 3 692 nected to the lead frames, WhlCh are thenimmersed 1n r a plating bath and revolved about the axis of the [52 us.Cl 134/158, 134/140, 134/149, frames and Within a hollow Cylindricalanode Spaced 1 156/345 midway between two flat anodes. A sparger pumpsthe [51 1m. (:1... B08b 3/08, BOSb 11/02 bath in a directionSubstantially perpendiclflar the [58] Field of Search." 134/157, 158159, 138440, axis of the frames. The frames and cylindrical anode134/149, 141, 153, 162; 156/345 are positioned midway between the topsurface of the bath and the bottom of a tank holding the bath. With [56]References Cited I this arrangement the frames are nonumformly plated,

with more old bern lated on the ends where bonds UNITED STATES PATENTSare to be n iade than Zn the carrier strips. Also, each a? 1 1 2 X;group of three leads of each frame for a single transistor is platedsubstantially alike. This arrangement of 332E111 231333:::;:.:1.-.---.-.-1:;;;...;;;;;.131/1311 tho fromoo ooo rotation of themin tho ooh also sults in the plating of a large number of frames using aminimum volume of a tank holding the bath. An increase in plating ratesis also achieved.

4 Claims, 7 Drawing Figures minnow SHEH l-llF 5 FIG-2 PATENTED 4 SHEET 3BF 5 Til 1 PATENTEnJuu 4 m4 SHEET [If 5 FIG-6 PATENTEDJun 4 can SHEEI S[If 5 1 NONUNIFORMLY TREATING ARTICLES AND SUPPORTING THEM THEREFORCROSS REFERENCE This application is a continuation-in-part of copendingapplication Ser. No. 263,897, filed June 19, 1972 now abandoned, whichis a division of application Ser. No. 100,176, filed Dec. 21, 1970, nowUS. Pat. No. 3,692,638.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to apparatus for nonuniformly treating articles and supportingthem therefor, and more particularly, to apparatus for plating anonuniform layer of a metal on a conductive article.

This invention is suited for use in the manufacture of semiconductivedevices such as transistors, diodes, integrated circuits or the like.While this invention is particularly suited for electroplating anonuniform gold layer on a nickel lead frame for transistors, it is alsoequally well suited for other chemical, mechanical or combined chemicaland mechanical treating applications, such as electroless plating,anodizing, polishing, cleaning or the like, wherein different degrees ofinteraction by the treating medium and the treated article are desired.

2. Description of the Prior Art In the manufacture of transistors, leadsfor the transistors are formed from a lead frame stamped from a sheet ofconductive metal such as nickel. Each lead frame has one group of threeleads for each transistor. Each group includes one outer lead having acurved end portion upon which a semiconductive chip is bonded, an-innerlead having a straight end portion upon which a fine wirev extendingfrom the chip is bonded, and another outer lead having another straightend portion upon which another fine wire extending from the chip isbonded.

The individual leads of each frame are joined together by a perforatedcarrier strip at one end so that the ends of the leads where bonding isto be performed are free of the strip. The leads are also joinedtogether intermediate-their ends by a relatively narrow support strip.Bothof the strips aresevered to separate the individual leads prior tothe completion of the ultimate transistor. (See Maguire, Koons andJarrett, Plastic Encapsulated Transistors, The Western ElectricEngineer, pp. 4l-5l, October, l970.)

Because an increased and controlled thickness of gold on the free endportions of the leads improves any bonds formed .on these ends, it isdesirable to have a greater thickness of gold on such ends than on theremainder. of the leads. Since chip bonds normally require a greaterthickness of gold to obtain good bonds than do wire bonds, it is alsodesirable that the curved free end portion of the leads have a greaterthickness of gold than the straight free ends of the leads. Since theperforated carrier strip is ultimately trimmed away,

. it is further desirable to have the least thickness of gold on it.While the leads themselves require some gold for thermal protection ofthe semiconductive chip and for corrosion protection and solderabilityof theleads, they do not require as much thickness of gold as do thefree ends where the chip and wire bonds are to be formed, but requiremore gold than the carrier strip. Such a nonuniform distribution of goldcan result in a very desirable efficient use and substantial savings ofgold.

In treating the lead frames by plating or similar techniques, it is alsodesirable to arrange themso as to treat a maximum number of frames at atime using a minimum volume. of a tank holding a treating medium. SuchSUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide new and improved apparatus for supporting articles in atreating medium.

Another object of this invention is the provision of apparatus fornonuniformly treating articles.

A further object of this invention is to provide apparatus fornonuniformly plating a layer of a metal on a plurality of conductivearticles.

This invention also contemplates apparatus for supporting articles in atreating medium. The apparatus includes a first member having aplurality of slots formed in the outer surface thereof for arranging thearticles in a spaced, radial, fan-like mannerfThe apparatus alsoincludes retaining members that cooperate with the first member formaintaining the articles in the cylindrical configuration.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention may be more clearly understood by referenceto thefollowing detailed description and drawings, wherein:

FIG. 1 is a greatly enlarged, fragmentary, perspective view ofa leadframe, showing leads joined together by a perforated carrier-strip and arelatively narrow support strip, showing semiconductive chips bonded tocurved portions of the free ends of outer leads, and showing. fine wiresextending from the chips to the other free ends of inner and other outerleads;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1, showing anonuniform layer of a metal on the lead frame of FIG. 1;

' FIG. 3 is a perspective view, partly in section, showing a tubularmember, having a plurality of spaced parallel slots, positioned withinand extending from a loading cup that assists the placing of the framesin the slots to arrange them in a cylindrical configuration;

FIG. 4 is an enlarged view of a portion of FIG. 2, showing the slots ofFIG. 2 in greater detail;

FIG. 5 is a front elevational view, partly in section, of the tubularmember of FIG. Shaving a plurality of lead frames mounted in slotsformed in its outer peripheral surface and immersed in a bath fornonuniformly electroplating gold on each frame;

FIG. 6 is a perspective view of an alternative embodiment of the presentinvention showing a plurality of lead frames mounted in a cylindricalconfiguration by inserting a flexible wire-like member alternately intoperforations formed in the carrier strips of the lead frames and intoconductive spacing washers; and

FIG. 7 is a perspective view of a preferred arrangement of the tubularmember of FIG. 3 with lead frames mounted thereto in a plating tankhaving a cylindrical anode, a pair of flat anodes and a sparger.

DETAILED DESCRIPTION LEAD FRAME Referring now to the drawings and inparticular to FIG. 1, a lead frame, designated generally by the numeral11, is shown. The .frame 11 has a plurality of groups of leads,designated generally by the numeral 12, one group being associated witheach transistor which is ultimately to be fabricated using the leadframe 11. Each group 12 includes an outer lead 13 having a curved endportion 14 to which a semiconductive chip 17 is bonded. Each group alsohas another outer lead 18 and an inner lead 21, both of which havestraight end portions 22 and 23 to which fine gold wires 24 extendingfrom the chip 17 are bonded.

Each lead framell includes a carrier strip 25 having perforations 26therein. The perforated carrier strip 25 joins the individual leads 13,18 and 21 together at one of their ends so that the opposite endportions 14,22 and 23where bonds are to be formed are free. The leads13, 18 and 21 are joined together intermediate their end portions 14, 22and 23 by a relatively narrow support strip 28.-

Typically, the lead frame 11 is stamped from a nickel sheet about milsthick. In the prior art a uniform layer of gold of about 0.255 to about0.315 mils was plated on the nickel lead frame 11.

In accordance with the present invention, a plurality of articles, suchas the lead frame 11 (FIG. 1), a substantially planar article or thelike, is nonuniformly treated. While the treatment may be any of anumber of different chemical or mechanical or combined chemical andmechanical treatments, such as electroplating, electroless plating,anodizing, polishing, cleaning or the like, the present invention willbe described in connection with electroplating. However, it is to beunderstood that treatments other than electroplating are within thespirit and scope of this invention.

More specifically, a nonuniform layer 29 (FIG. 2) of a metal, such'asgold or the like, is electroplated on the lead frame 11 (FIGS. land 2)so that the layer 29 has the leastthickness on-the carrier-strip 25, agreater thick'nesson the leads '13, 18 and 21, a still greater thicknessat the straight free end portions 22 and 23 where wire bonding is to beperformed, and the greatest thickness at the curved free end portionl4where chip bonding is to be performed.

LOADING In carrying out the present invention, into a cup 30 (FIG. 3)there is placed an assembly including a base 31, a rotatable shaft 32fixed to and extending from the top side of the base 31 and a hollowtubular member 33 with its bottom'end also fixed to the top side of thebase 31. The placing of this assembly in the cup'30-positions the bottomside of the base 31 in a central de pression 42 in the cup 30 andextends the shaft32 and the tubular member 33 from the cup 30 and beyondits side walls. I

As shown in FIGS. 3 and 4, the hollow tubular member 33 has a pluralityof spaced slots 46 formed in its outer surface that are parallel to eachother, and to the shaft 32 and the axis of the member 33. The bottomends of the slots 46 are closed by an annular indentation 49 formed inthe top side of the base 31.

Typically, the slots 46have a width of about l5 to 20 mils so that theyeasily receive the lead frames 11 with a thickness of about 10 mils. Theinner side portions47 (FIGS. 1 and 3) of a plurality of lead frames 11are individually placed in the slots 46.By so placing these frames 11,they are arranged in a substantially radial, fan-like manner and in ahollow cylindrical configuration, as shown in FIG. 3. In thisconfiguration the outer side portions 48 of the frames 11 are located onthe outer periphery of the'configuration and are spaced more from eachother than the inner side portions 47, as is apparent from FIG. 3. Sincethe bottom ends of the slots 46 are enclosed by the annular indentation49 in thebase 31, the frames 11 are axially retained in the slots 46 toprevent them from dropping out. Also, the frames 11 are laterallyrevices, such as machine screws 56 or the like.

By so mounting the capping member 52, the top end of the tubular member33 fits into the annularindentation 54 and closes the top ends of theslots 46 to axially retain the frames 11 in the slots 46. Also, a rim 57formed by the indentation 54 laterally retains the frames 11 in theslots 46. Thus, the combination of the slots 46 in the tubular member33, the indentations 49 and 54 in the base 31 and in capping member 52,and the rims 51 and 57in the base 31 and in capping member 52 completelysupport and retain the frames 11 in the cylindrical configuration.Therefore, after the capping member 52 is mounted to the tubular member33, the loading cup 30 is no longer necessary to completely support andretain the frames 11 in the cylindrical configuration.

TREATING MEDIUM member 52 are removed from the loading cup 30 by liftingsuch assembly by the shaft 32 from the cup 30. The shaft 32 is thenconnected to a conventional rotating device, such as a motor 61 (FIG.5).

The assembly is next immersed in a treating medium with the shaft 32extending therefrom'by suspending the motor 61 from a'fixed hook 62 byan eyelet 63 fastened to the motor 61. Also, the motor 61 is providedwith a handle 64 to facilitate the manipulation of the motor and theassembly connected thereto. While the assembly is shown in FIG. 5 asbeing suspended vertically by the hook 62 andthe eyelet 63, it should beunderstood that the assembly may be positioned other than vertically inthe treating medium by the use of conventional supporting devices. Suchnonvertical positioning does not reduce the effectiveness of theinvention.

Illustratively, the present invention is used for electroplating anonuniform layer of a metal suchas gold on a conductive article such asa nickel lead frame 11.

and potassium gold cyanide. Also, it may be desirable to preplate theframes '11 in a plating solution based on potassium phosphate with lowconcentrations of potassium gold cyanide.

In addition to the assembly, the electroplating bath 65 contains ananode 67 of a plating source 68 which also has a cathode 69 connected tothe shaft 32 by con- ;entional expedients such as a brush 71 and a slipring I In order to deposit'the nonuniform layer 29 (FIG. 2) of gold onthe lead frames 11 in accordance with the present invention, the motor61 (FIG. 5) is energized to revolve the lead frames 11 past the anode67. The

power source 68 isthen activated to pass current from such source 68 tothe anode 67, the bath 65, the lead frames 11, the walls of the slots 46of the tubular member 33, the remaining structure of the tubular member33, the base 31, the shaft 32, the slip ring 72, the brush 71 to thecathode 69 and back to the source 68.

By revolving the frames 11 in the bath 65 past the anode 67 andactivating the power source 68, gold is plated from the bath 65 onto thelead frames 11 to form the nonuniform gold layer 29 (FIG. 2) thereon.The gold layer 29 has its greatest thickness at the outer side portions48 of the lead frames 11, which include the curved end portion 14 andthe'straight end portions 22 and 23, and has its least thickness at theinner side portions 47 of the frames 11, which include the perforatedcarrier strip 25. Further, the layer 29 has a thickness on the leads 13,18 and 21 greater than that onthe carrier strip 25, a still greaterthickness on the straight end portions 22 and 23 where the wires 24and25 are to be bonded, and its greatest thickness on the curvedend portion14 where the chip 17 is to be bonded.

The gold layer 29 is thicker at the outer side portions 48 of the frames11 than at theinner side portions 47 of the frames 11, because, it isbelieved there is a greater interaction of the portions 48 with theplating bath 65 than there is of the portions 47 with such bath 65. Morespecifically, it is believed that the layer 29 .is thicker at theportions 48 than at the portions 47 because the portions 48 n have the.greatest velocity through the plating bath 65, are spaced further apartand are exposed to a greater undepleted amount of the bath 65. Also,theportions 48 are closer to the anode 64 than the portions 47 when maximumplating occurs.

(See the lefthand side of F IG. 5.) Further, because the portions 47 arelocated in the slots 46, these portions 47 are exposed to a smalleramount of the undeple'ted bath 65 and the slots 46, tend topartiallymask the po tions 47. This aids in reducing the thicknessof the goldlayer 29 on the portions 47. f; r v

Typically, the thickness of the nonuniform gold layer 29 on the frames11 varies linearly with distancefrom approximately 0.300 mils at thecurved end portion14j decreasing to approximately 0.060 mils at thecarrier strip 25. Such thickness of the layer 29 has been achieved witha plating current of about 30 amps and a rotational speed of 50 rpm. Thedegree to which the thickness of the deposited gold layer is nonuniformis controlled by such factors as the rotational speed of the frames inthe bath 65, the spacing of the frames 11, the plating current, and the.composition of the plating bath.

By plating the lead frames 11 in accordance withthe present invention,substantial savings in gold result and at the same time adequate amountsof gold are plated 0n the curved end portion 14 of the lead 13 formaking good chip bonds and on the straight end portions 22 and 23 of theleads 18 and 21 for making good wire bonds. Also, by arranging the leadframes 11 in the cylindrical configuration a'large number of frames 11can be simultaneously plated using a small-volume tank holding theplating bath. In addition, the plating time can be decreased over priorart techniques and at the same time the treating rate and control overthe treating operation is increased.

To prevent gold from being plated from the bath 65 on the shaft 32, thebase 31, and the tubular member 33, which are electrically conductive;these element 32, 31 and 33 are completely coated with anonconductive'film, such as that sold under the trademark Kynar, byPennwalt. However, the walls of the slots 46 which must make electricalcontact with the frames 11 to plate them are not coated. The shaft 32 isalso coated with the film except where the slip ring 72 is attached. Thecapping member 52 is made of an electrically insulative material andtherefore, no gold is plated from the bath on it. The walls of the slots46 receive a minimum thickness of plated gold, since the frame 11 areinserted in the slots 46 exposing only a small amount of the walls ofthe slots 46 to the plating bath 65, since the slots 46 are positionedrelatively far away from the anode 64, and since the slots 46 receive arelatively low velocity through the bath 65. After being used numeroustimes to plate large quantities of frames 11, any gold plated on thewalls of the slots 46 can be chemically stripped therefrom.

Instead of using the plating bath arrangement of FIG.

5, that of FIG. 7 may be used to nonuniformly plate the lead'frames 11.The arrangement of FIG. 7 has the ad- .ditional advantageof plating eachgroup 12 of each frame 11 with greater uniformity from group to'groupthan is achievable with the arrangement of FIG. 5.

Referring now to FIG. 7, a hollow cylindrical anode 91 is shownsupported on three equally spaced legs 92. These legs 92 are positionedon the bottom of a tank 93 that holds the plating bath 65. The tubularmember 33 with the frames 11 mounted in the slots 46 thereof (as shownin F IGS, 3 and 5) and with the capping member 52mounted on the tubularmember 33 is centrally positioned within the anode 91, as shown in FIG.7. The anode 91 is positionedsubstantially midway between the uppersurface of the bath 65 and the bottom of the tank 93 and substantiallymidway between a pair of flat anodes 94. The anode 91 is alsoelectrically connected to these anodes 94 by bars 95. The anodes 94 are,in turn, connected to the positive terminal of the power supply 68. Asin the arrangement of FIG. 5, the cathode 69 of the power supply 68 isconnected to the frames 11 by way of the brush 71 and slip ring 72.

Also positioned on the bottom of the tank 93 is a sparger 97 throughwhich a portion of the bath 65 is forced by a pump 98 after it has beendrawn from an orifice 101 and passed through a filter 99. The directionof flow of the efiluent from the sparger 97 is substantiallyperpendicular to the shaft 32.

' ALTERNATIVE FRAME SUPPORTING ARRANGEMENT An alternative framesupporting arrangement of the invention is shown in FIG. 6. Morespecifically, the lead frames 11 are formed in a hollow cylindricalconfiguration for immersion in the plating bath 65 by first inserting awire-like member 76 alternately into the uppermost perforations 26 ofthe carrier strips 25 of the lead frames 1 1 and then into a pluralityof conductive washers 77 spaced from each other by the frames 11.

Also, another wire-like member 78 is inserted alternately into thelowermost perforations 26 of the carrier strips 25 and into a pluralityof conductive washers 79 spaced from each other by the frames 11, asshown in FIG. 6.

Next, the wire-like members 76 and 78 are formed into a substantiallycircular configuration. This results in holding the frames 11 in asubstantially radial, fanlike manner and in arranging of the lead frames11 in a hollow cylindrical configuration, as shown in FIG. 6. lnthisconfiguration the outer portions 48 of the frames 11 are located on theouter periphery of the configuration and are spaced more from each otherthan the inner side portions 47, asis apparent from FIG. 6.

An insulative disc element 80 is positioned in the center of the hollowconfiguration of the lead frames 11 adjacent the uppermost perforations26 of the carrier strips 25. The ends of the wire-like member 76 arethen joined together to force the frames 11 and washers '77 against theelement 80, as shown in FIG. 6. Next, the ends of the wire-like member76 are fastened with a conventional holding expedient 81 to retain themember 76 in a ring-like configuration.

A conductive base plate 82 having a conductive disc element 83 fixedthereto is then positioned beneath the lead frames 11 in electricalcontact with them and the washers 79 so that the element 83 is in thecenter of the cylindrical configuration, as shown in FIG. 6. The ends ofthe wire-like member 78 are next joined together to force the frames 11and the washers 79 against the disc element 83. The ends of thewire-like member 78 are then'fastened with a conventional holdingexpedient anode 67 to plate gold from the bath 65 on the frames 11 inthe same way that gold was plated on the frames 11 in the embodiment ofFIG. 5.

It is to be understood that the above-described arrangements are simplyillustrative of the application of the principles of this invention.Numerous other arrangements may be readily devised by those skilled inthe art which will embody the principles of the invention and fallwithin its spirit and scope.

What is claimed is:

1. An apparatus for treating planar articles, which comprises: v

a member having a plurality of slots formed in the outer surface thereoffor loosely receiving the pla nar articles with portions thereofextending out of the slots and being external relative to the surface ofsaid member, and for arranging such articles in a spaced, radial,fan-like manner about the axis of said member, such axislying intheplanes of the articles for partially masking the inner side portions ofthe articles;

- means cooperating with the member for retaining the articles in saidmanner; means for positioning the articles in a treating medium; and vmeans for moving the articles positioned in the treating medium aboutthe axis to increase the interaction of the outer side portions of thearticles with the medium to nonuniformly treat the articles. 2. Theapparatus of claim 1, wherein the outer surface of the member is convex.

3. An apparatus for treating planar articles, which comprises:

means for loosely receiving the planar articles and for arranging saidarticles in a spaced, radial, fan-like manner about an axis with theaxis lying in the planes of the articles, for partially masking theinner side portions of the articles and for spacing the articles fromeach other; 7 means, cooperating with the receiving and arranging means,for retaining the articles in said manner; means for positioning thearticles in a treating medium; and means for moving the articlespositioned in the heating medium about the axis to increase theinteraction of the outer side portions of the articles with the mediumto nonuniformly treat the articles. 4. The apparatus of claim 3, whereinthe retaining means includes a pair of cooperating members havingannular channels for holding end portions of the articles.

1. An apparatus for treating planar articles, which comprises: a memberhaving a plurality of slots formed in the outer surface thereof forloosely receiving the planar articles with portions thereof extendingout of the slots and being external relative to the surface of saidmember, and for arranging such articles in a spaced, radial, fan-likemanner about the axis of said member, such axis lying in the planes ofthe articles for partially masking the inner side portions of thearticles; means cooperating with the member for retaining the articlesin said manner; means for positioning the articles in a treating medium;and means for moving the articles positioned in the treating mediumabout the axis to increase the interaction of the outer side portions ofthe articles with the medium to nonuniformly treat the articles.
 2. Theapparatus of claim 1, wherein the outer surface of the member is convex.3. An apparatus for treating planar articles, which comprises: means forloosely receiving the planar articles and for arranging said articles ina spaced, radial, fan-like manner about an axis with the axis lying inthe planes of the articles, for partially masking the inner sideportions of the articles and for spacing the articles from each other;means, cooperating with the receiving and arranging means, for retainingthe articles in said manner; means for positioning the articles in atreating medium; and means for moving the articles positioned in theheating medium about the axis to increase the interaction of the outerside portions of the articles with the medium to nonuniformly treat thearticles.
 4. The apparatus of claim 3, wherein the retaining meansincludes a pair of cooperating members having annular channels forholding end portions of the articles.